It is a well-known technique to reproduce an image by photographic procedures using a photosensitive composition comprising a polymerizable compound having an addition polymerizable unsaturated bond, a photopolymerization initiator and, if desired, an appropriate film-forming binder and a thermal polymerization inhibitor. A photosensitive composition of this type undergoes photopolymerization on exposure to light to be cured and insolubilized as described, e.g., in U.S. Pat. Nos. 2,927,022, 2,902,356, and 3,870,524. Light exposure of the photosensitive composition in a film form through a desired negative followed by removal of the unexposed area with an appropriate solvent (hereinafter referred to as development) gives a desired cured image of the polymerized composition. Thus, the photopolymerizable composition of this type is extremely useful for the production of printing plates and the like.
Since a polymerizable compound having an addition polymerizable unsaturated bond, when used alone, does not exhibit sufficient photosensitivity, it has been proposed to use a photopolymerization initiator in combination to improve photosensitivity. Conventionally employed photopolymerization initiators include benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, and 2-ethylanthraquinone, etc. However, photopolymerizable compositions using these photopolymerization initiators have low response on curing and therefore require a long time for imagewise exposure for image formation. For this reason, if a vibration, even slight, takes place during exposure, a minute image cannot be precisely reproduced. Further, consideration should be given to dissipation of a large quantity of heat accompanying the increased energy emitted from a light source for exposure. In addition, the heat generated tends to cause deformation or denaturation of the photosensitive film.
Moreover, the conventional photopolymerization initiators are considerably less capable of photopolymerization with light in the visible region of 400 nm or more as compared with their ability with light in the ultraviolet region of 400 nm or less. Therefore, photopolymerizable compositions containing the conventional photopolymerization initiators have been of very narrow application.
Several proposals with respect to photopolymerizable systems sensitive to visible light have hitherto been made. For example, U.S. Pat. No. 2,850,445 reports that a certain kind of photoreducing dyes, such as Rose Bengale, Eosine, and Erythrocin, exhibit effective sensitivity to visible light. Also included in improved techniques are a dye/amine complex initiator system (JP-B-44-20189, the term "JP-B" as used herein means an "examined published Japanese patent publication"), a hexaarylbiimidazole/radical generator/dye system (JP-B-45-37377), a hexaarylbiimidazole/p-dialkylaminobenzylidene ketone system (JP-B-47-2528, JP-A-54-155292, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a 3-keto-substituted coumarin compound/active halogen compound system (JP-A-58-15503), and a substituted triazine/merocyanine dye system (JP-A-54-15102). These techniques proposed are certainly effective with respect to the photopolymerization rate to visible light to some extent, but are still not satisfactory, and a further improvement has been demanded.
On the other hand, studies having been recently directed to increase in sensitivity to ultraviolet light and image formation using a laser beam, UV projection exposure, laser direct plate making, laser facsimiles, holography, etc. have already been put into practical use. In order to cope with these latest techniques, photosensitive materials having high sensitivity have now been under development, but sufficient sensitivity has not yet been reached.